The invention relates to a process for improving the strength properties of partially desulfurized coke, especially petroleum coke which can be used to manufacture electrodes, in particular anodes for the Hall-Heroult fused salt electrolytic process for the production of aluminum. The process according to the invention comprises an after-treatment for partially desulfurized, calcined coke for at least 30 minutes at an elevated temperature, as a result of which there is an increase in the strength of the coke.
In the aluminum industry carbon based materials are employed for the production of aluminum, in particular anodes made out of calcined coke residue or green cokes which contain large amounts of sulfur. Whereas green coke containing about 3 wt.% sulfur was hardly ever used by the aluminum industry up to a few years ago, such coke is now widely used for reasons of costs and/or due to the shortage of better quality coke.
Cokes for the production of anodes for the aluminum smelters, often because of environmental laws but also because of cost factors related to the electrolytic process, must contain less than about 2 wt.% sulfur.
There has been no lack of efforts to develop processes for desulfurizing high sulfur cokes so that these cokes will, in terms of sulfur content, satisfy the necessary conditions, in particular those concerning the environment. In most countries the environmental laws permit SO.sub.2 emissions in amounts which correspond to a concentration of 1.8 wt.% sulfur in the atmosphere. In the first phase of developments single stage processes were proposed, whereby the sulfur content was reduced to the required level by directly heating the green coke to temperatures of up to and over 1500.degree. C.
However coke is required to satisfy other, important requirements if it is to be made into anodes for aluminum reduction cells. For example, the density and the physical strength should be as high as possible; on the other hand the reactivity towards CO.sub.2 or air should remain sufficiently low. Also of importance is the crystallinity, electrical conductivity and purity.
It is therefore not surprising that various calcined cokes, which have been produced by processes aimed solely at lowering the sulfur content, were not able to satisfy these requirements.
In the following art processes which are mostly of the 2-stage kind, there results in a quality of coke which is suitable for anode manufacture. In the 1st stage, frequently in the temperature range below 1000.degree. C., measures are taken to effect only insignificant desulfurization, and in the 2nd stage the final calcination of the coke takes place producing the required reduction in the sulfur content, at the same time however fulfilling some part of the requirements with respect to the above mentioned properties. Therefore, for example, according to the German published patent application 29 03 884 about 70% of the volatile constituents is removed in the first stage in the temperature range of .about.490.degree. C. to approximately 850.degree. C., and in the second stage calcination is carried out at a temperature of at least 1500.degree. C. so that the largest part of the sulfur is removed without substantially altering the bulk density of the coke. This process is in the first instance directed only at the bulk density of the coke, and ignores the essential, important property of physical strength of the coke. It can be said in general about the 2-stage process for desulfurizing green cokes that the procedures are very involved, which results e.g. in the quality fluctuating strongly from charge to charge. Another disadvantage is the much higher price of the end product compared with that from single stage processes.